1. Field of the Invention
The invention relates generally to the field of nuclear magnetic resonance (“NMR”) measurement of subsurface formations penetrated by a wellbore. More specifically, the invention relates to methods and apparatus for measuring NMR free induction decay signals using wellbore deployed instruments, and the use of such free induction decay signals in determining NMR properties of the formation.
2. Background Art
NMR instruments used to make measurements of NMR properties of subsurface Earth formations, called “well logging” tools or instruments are moved along the interior of a wellbore drilled through such formations. NMR well logging devices known in the art include one described in U.S. Pat. No. 5,055,787 issued to Kleinberg et al. and assigned to the assignee of the present invention. As a general principle, the instrument disclosed in the '787 patent makes measurements by inducing a strong, substantially homogeneous static magnetic field B in a volume of an adjacent subsurface formation on one side of the instrument to measure nuclear magnetic resonance characteristics thereof. The instrument has a radio frequency (“RF”) antenna mounted on the outside of a metal body of the instrument, directing focused oscillating RF magnetic fields at the volume to tip the magnetic moments of hydrogen nuclei of fluids within pore spaces in the subsurface formations. The same antenna can be used to receive signals of proton precession emanating from within the volume of interest after transmission of the RF polarizing field. Rapid damping of the antenna energy between the transmitting and receiving modes of operation is accomplished by a Q-switch. The disclosed instrument provides for the direct measurement of NMR signal decay having transverse relaxation time (T2) behavior, and further provides for the fast repetition of pulsed measurements from within a wellbore. An additional magnet array may be mounted offset from the first magnet configuration to prepolarize a formation before it is measured in order to pre-align a larger number of hydrogen nuclei than a single magnet configuration could do by itself.
The instrument described in the '787 patent, as is the case for other NMR well logging instruments known in the art, makes measurements of transverse relaxation time properties of the subsurface formations using a pulsing sequence known as Carr-Purcell-Meiboom-Gill (“CPMG”), or modifications of the CPMG sequence. The CPMG sequence is initiated after hydrogen nuclei are prepolarized along the direction of the static magnetic field by applying an RF field having frequency substantially equal to the Larmor frequency of the hydrogen nuclei, and amplitude and duration selected to reorient the nuclear magnetic spin axes of the hydrogen nuclei to be transverse to the static magnetic field direction (called a 90 degree pulse). Proton spin precession about the static magnetic field direction induces signals in the RF antenna that are detected by the instrument and called the Free Induction Decay (FID). Over a period of time, the nuclear magnetic spins of the hydrogen nuclei become out of phase with each other, such that the detected RF magnetic field signal decays substantially to zero. After a selected time interval, a series of “refocusing” pulses is applied. The refocusing pulses have duration and amplitude selected to invert the spin phasing of the hydrogen nuclei so that eventually the proton precession will come back in phase. When the proton precession comes back in phase, an RF signal is induced in the RF antenna and is detected. Such signal generation and detection is referred to as “spin echo” detection. The refocus pulsing and RF spin echo signal detection is repeated for a selected number of pulses. Each successive spin echo is reduced in amplitude from the preceding one. The rate at which the spin echo amplitude decays is related to the transverse relaxation time (T2) properties of the various fluids in the subsurface formations. Analysis of the fluids in the formation may be performed by analyzing the multicomponent exponential decay of the amplitudes of successive spin echoes. A result of such analysis is a T2 distribution of the various hydrogen-bearing fluids in the subsurface formations. Such distribution may be related to the petrophysical properties of the formations.
If a pure FID signal is measured, the same analysis can be applied to the FID signal. The FID signal can be correlated to useful information, such as the fractional volume of fluid filled pore spaces (porosity) in the subsurface formations. Notwithstanding that the above mentioned NMR apparatus induces a substantially homogeneous static magnetic field in the formations, there is still some inhomogeneity in the static magnetic field. Such inhomogeneity is an essentially unavoidable result of “inside out” NMR apparatus such as well logging instruments, wherein the volume of investigation is entirely outside the apparatus. The inhomogeneity of the static magnetic field has the effect of shortening the FID signal decay time so that its measurement becomes impracticable. Also it is difficult to design a magnet for application with well logging instruments having a magnetic field within a few parts per million (ppm) homogeneity within the investigated regions.
Another NMR property of interest is the longitudinal (T1) relaxation time. Techniques known in the art for measuring T1 of subsurface formations include a technique that determines a T1/T2 ratio using multiple waiting times between successive CPMG sequences. Such technique is described in U.S. Pat. No. 5,486,742 issued to Freedman et al. and assigned to the assignee of the present invention. A characteristic common to T1 determination techniques known in the art is that multiple pulse sequences are used, whether having a single wait time between sequences or otherwise. The length of time to acquire such sequences has the practical effect of limiting the speed at which the well logging instruments can be moved through the wellbore.
There exists a need for techniques that enable measurement of the FID signal by an NMR well logging instrument, and techniques to increase the effective logging speed while measuring T1 properties of the subsurface formation.